This invention relates to a microwave oven with a mechanical locking assembly, and in particular to a control circuit for the microwave oven.
A conventional microwave oven has a mechanical switch locking assembly, which permits operation of the microwave oven via the start switch only when the door is closed. This type of the microwave oven is shown, for example, in Japanese Utility Model No. Sho 54-38307 published on Nov. 14, 1979, incorporated herein by reference.
FIG. 1 shows a control circuit of the microwave oven with the mechanical locking assembly of the prior art. As shown in FIG. 1, a cooking start switch 1 is provided between a power source 2 and a high voltage transformer 3. The transformer 3 produces a high operating voltage for magnetron 4. The cooking start switch 1 is operated in response to depression of a cooking start button (not shown) and is turned on mechanically in the mechanical locking assembly which includes the door latching assembly. Namely, cooking start switch 1 turns on when the cooking start button is pushed but only if the microwave door is closed, and the oven is turned off if the door is opened. A door switch 5 and a door lock monitor switch 6 are turned respectively on and off, when the door is closed and locked.
A mechanical timer motor 7 turns a timer switch 8 off after a predetermined cooking time is over. The control circuit further includes additional parts, such as a magnetron thermal switch 9, a fuse 10, a blower motor 11, and a cooking lamp 12.
Accordingly, if the door is closed when food is put into the oven, door switch 5 and door lock monitor switch 6 are respectively locked to an on (closed) and off (open) positions. Timer switch 8 is turned on (closed) when the cooking time is set by a timer setting device (not shown). The cooking start switch 1 may then be turned on to energize the magnetron 4.
The magnetron 4 produces sufficient microwave energy for cooking. If the door is opened during the cooking operation, cooking start switch 1 is turned off, and magnetron 4 is deenergized to cancel the cooking operation. The cooking time does not lapse, since the timer motor 7 is de-energized. If the door is once again closed, the start switch 1 may be turned on to once again energize the magnetron 4 to resume the cooking operation.
However, in the conventional microwave oven with the mechanical locking assembly, if the power source 2 is connected when the cooking start switch 1 is turned on, magnetron 4 is energized directly. Therefore, it may be dangerous to energize magnetron 4 directly as, for example, when there is no food in the oven since there is a danger of fire or sparking due to high energy reflections.